The present invention relates to persistent p-type group II-VI semiconductor compounds, including fabrications methods and devices containing such compounds.
As used herein, the group II-VI semiconductor compounds include group II elements selected from zinc, cadmium, the alkaline earth metals such as beryllium, magnesium calcium, strontium, and barium, and mixtures thereof, and group VI elements selected from oxygen, sulfur, selenium, tellurium, and mixtures thereof. The group II-VI semiconductor compounds may be doped with one or more p-type dopant. Such p-type dopants include, but are not limited to, nitrogen, phosphorus, arsenic, antimony, bismuth, chalcogenides of the foregoing, and mixtures thereof. Zinc oxide and zinc sulfide are two presently preferred group II-VI semiconductor compounds.
Zinc oxide (ZnO) and zinc sulfide are wide band gap semiconductors with potential for use in electrically excited devices such as light emitting devices (LEDs), laser diodes (LDs), field effect transistors (FETs), photodetectors operating in the ultraviolet and at blue wavelengths of the visible spectrum, and other similar devices. Gallium nitride (GaN) and gallium arsenide (GaAs) are commonly used as a semiconductor material for the electronic devices mentioned above.
Zinc oxide has several advantages over GaN. For instance, ZnO exciton binding energy is 60 meV, which is about three times greater than GaN, and which suggests that ZnO-based lasers should have more efficient optical emission and detection. Zinc oxide drift mobility saturates at higher fields and higher values than GaN, potentially leading to higher frequency device performance. The cost and ease of manufacture of zinc oxide is attractive when compared to other common semiconductor materials. Zinc oxide has superior radiation-resistance (2 MeV at 1.2×1017 electrons/cm2) compared to GaN, which is desirable for radiation hardened electronics. Zinc oxide has high thermal conductivity (0.54 W/cm·K). Zinc oxide has strong two-photon absorption with high damage thresholds, rendering it ideal for optical power limiting devices. Zinc oxide forms two stable polytypes: wurtzite and zincblende; however, polytypism is not as prevalent as with GaN, AlN, and SiC.
N-type zinc oxide semiconductor materials are known and relatively easy to prepare, such as ZnO doped with aluminum, gallium, or other known n-type dopants. P-type zinc oxide semiconductor materials are theoretically possible, but have been difficult to prepare. D. C. Look et al., “The Future of ZnO Light Emitters,” Phys. Stat. Sol., 2004, summarize data on p-type ZnO samples reported in the literature. The best reported ZnO samples have resistivity values of 0.5 ohm·cm (N/Ga dopants) and 0.6 ohm·cm (P dopant). Many of the reported p-type zinc oxide samples tend to be light, heat, oxygen, and moisture sensitive. Some convert to n-type material over time. Their stability has been questioned and instability has been observed. Some of the more-stable p-type zinc oxide materials reported in the literature are prepared using RF sputtering fabrication processes. No commercially viable p-type zinc oxide semiconductor materials are currently known.
Without being bound by theory, it is presently believed one possible explanation for the lack of p-type zinc oxide materials is because high temperature diffusion processes or other fabrication methods inhibit formation of desirable p-type zinc oxide compounds.
Fabrication temperature determines the effective limits for diffusion to be applicable in certain semiconductor systems. At low temperature the process is limited by slow diffusion, and when the temperature becomes sufficiently high for diffusion to occur, chemical reaction between the p-type dopant and oxygen forms stable gaseous species that make the p-type zinc oxide semiconductor structure unstable.
With reference to p-type zinc oxide, the p-type dopant may substitute for oxygen or substitute for zinc. If the dopant replaces oxygen, then it would be an anion, and if the dopant replaces zinc, it would be a cation. Referring to arsenic as the p-type dopant, there are two substitutional defects that are considered as the most probable (AsO) and (AsZn). The notation “AsO” refers to an arsenic atom replacing an oxygen atom in the ZnO lattice. The notation “AsZn” refers to an arsenic atom replacing a zinc atom. It is believed that both AsO and AsZn result in the formation of complex crystal lattice coordination compounds that may include the formation of zinc vacancies on zinc sites, shown by the notation “VZn”.
It has recently been suggested by researchers Sukit Limpijumnong et al. that large-size-mismatched zinc oxide dopants, such as arsenic and antimony, replace zinc in the crystal lattice, forming the complex AsZn-2VZn. Sukit Limpijumnong et al., “Doping by Large-Size-Mismatched Impurities: The Microscopic Origin of Arsenic- or Antimony-Doped p-Type Zinc Oxide,” Phys. Rev. Lett., Vol. 92, No. 15, Apr. 16, 2004. Based upon first-principles total energy calculations, these researchers concluded that it is “impossible for AsO to dope ZnO efficiently p type.”
Without being bound by theory, it is presently believed that stable, persistent, p-type group II-VI compounds are fabricated under conditions in which the p-type dopant, such as arsenic, substitutes for the group VI element, such as oxygen. Indeed, without being bound by theory, it is presently believed that the failure to prepare persistent p-type group II-VI compounds may often be explained because of the p-type dopant is doped into the compounds under conditions where it has a positive oxidation state (cation), replacing the group II element in the crystal lattice. Therefore, it would be an advancement in the art to provide persistent p-type group II-VI semiconductors generally, and persistent p-type zinc oxide specifically, and to provide fabrication methods and devices containing such compounds.